Tensioning system for an orthodontic outer brace

ABSTRACT

A tensioning system for an orthodontic outer brace includes a resilient element and coupler for transmitting a tensile force exerted by the resilient element to the outer brace. The tensioning system involves an arc-shaped guide path, along which the resilient element is freely movable. The resilient element preferably has a spring constant which decreases or is zero on further extension after a specific spring force or extension is reached. The resilient element is preferably a helical spring.

This application is a division of application Ser. No. 08/464,895, filedJun. 29, 1995, now U.S. Pat. No. 5,667,380.

DESCRIPTION

The present invention relates to a tensioning system for an orthodonticouter brace, comprising resilient means, and coupling means fortransmitting a tensile force exerted by the resilient means to the outerbrace.

Tensioning systems of this type are generally known, for example fromU.S. Pat. No. 3,526,035. This patent discloses a tensioning system for aso-called neck brace, comprising a neck band on which a strip offlexible material is mounted. Two small protective tubes are fixed tosaid strip, there being a spring in each tube. The springs are fixed onone side to the carrying strip and on the other side to one end ofcoupling means, which can be fixed to the outer brace by their otherend. By stretching the respective springs and then fixing the couplingmeans to the outer brace in such a way that the springs remainstretched, a tensile force directed towards the neck is exerted on theouter brace. Via an inner brace fixed to the outer brace at the mouth,said tensile force is transmitted to two or more teeth, by which meansthe position of the latter can be corrected.

A further orthodontic outer brace having a tensioning system isdisclosed in DE-A 3 417 256. In this case the neck band is slidablymounted in plastic guide sleeves. Said neck band is provided at bothends with perforations for adjustably fixing ends of resilient elements(in the form of elastic strips), which can be fixed at the opposite endto the outer brace itself. Resilient elements of this type attached toeither side of the head between the ends of the neck band and the endsof the outer brace require a certain fitting length and fixing elementswhich seriously impede any freedom of head movement which the wearer mayhave. Moreover, accurate adjustment of a desired tensioning force forcorrection of the teeth is made more difficult by the interaction of thetwo resilient elements joined to one another by the neck band.

The comfort of tensioning systems of this type for the wearer leavessomething to be desired because the freedom of movement of the head isimpeded. If the patient turns his or her head to the left or the right(that is to say shakes his or her head to indicate no), an additionalcompressive force directed towards the left side or the right side ofthe teeth and backwards (that is to say towards the neck) will beexerted on the teeth concerned, whilst on the other side of the teeth atensile force directed frontwards will be exerted on the teeth concernedwhich are located on the right or on the left. These additional forcescan be fairly large, are unpleasant for the patient and are undesirablefrom the standpoint of the orthodontic treatment. Similar problems arisein the case of the movement of the head to indicate yes, in which casethe resilient means are stretched further.

The aim of the present invention is to provide a tensioning system foran orthodontic outer brace with which the tensioning force to betransmitted to the teeth remains as constant as possible and the comfortfor the wearer and the freedom of movement of the patient are increased.

According to the invention, this aim is achieved in that the resilientmeans comprise a resilient element, in that the tensioning systemcomprises an arc-shaped guide path, along which the resilient element ismovable, and in that the coupling means can be fixed on one side to theouter brace and are connected to one another on the other side via theresilient element. As a result of these measures, the resilient elementmoves along the guide path when the patient turns his or her head. Withthis arrangement, the resilient element, such as, for example, anelastic band or a coil spring, is, as it were, fitted between two pairsof coupling means, such as, for example, two pieces of cord-like orstrip-like material, in such a way that if a pull is exerted on onecoupling means, a tensile force is transmitted via the spring to theother coupling means. The spring and the coupling means are able to movefreely along the guide path in the direction of the guide path. If thetensioning system is fitted to a patient's neck and the coupling meansare fixed to the outer brace, the entire unit, comprising the arms ofthe outer brace, the coupling means and the resilient element, forms, asit were, a closed "ring", which runs around the neck, the cheeks and themouth of the patient. If the head is moved to the left or to the right,the "ring" turns in concert via the mouth and the outer brace, and theresilient element guided along the guide path will therefore move aroundin the same direction. With this arrangement, the resilient element willbe subjected to no or hardly any additional stretching, as a result ofwhich the forces exerted on the teeth remain virtually constant. Urgepeaks in these compressive forces, such as occur with conventionaltensioning systems, are largely avoided.

In order to protect the movable resilient element and to guarantee thefree movability thereof, it is advantageous, according to the invention,for the guide path to comprise a tube. An additional advantage is thatthe patient's hair does not get caught in the resilient element and doesnot impede the movement backwards and forwards along the guide path.

To ensure optimum comfort in wear and to ensure that the resilientelement is movable as flexibly as possible, it is advantageous,according to the invention, if the guide path is flexible, in such a waythat the arc shape thereof can be adjusted to the curvature of thepatient's neck. In this context, it is particularly advantageous if thearc shape, as it were, continually adjusts to the shape of the neckduring use.

According to the invention, a flexible tubular guide path can beadvantageously obtained by shaping the guide path from a wire or stripwound to give a spiral-like winding. However, ordinary tubes made of aflexible material are also very suitable.

In order to prevent, for example, the patient's hair from getting caughtbetween the spiral-like winding it is advantageous, according to theinvention, if the outside of the spiral-like winding, is provided with acovering which covers the winding.

Very supple movement of the resilient element is achieved, according tothe invention, in that the guide path comprises an arc-shaped strip,preferably made of metal, which has an essentially smooth sliding planefor the resilient element. The cross-sectional shape of the slidingplane can, if necessary, be matched to the cross-sectional shape of theresilient element. The arc-shaped strip then forms a guide channel whichhas a sliding plane on the inside.

The resilient element can move along the guide channel with very littlefriction. Wire-like coupling means made of plastic, such as nylon, willmake contact with the guide path because of the arc shape of the latter,so that there will be some question of friction. This friction is verysmall if the guide path has a smooth metal sliding plane.

In the case of a tubular guide, the internal dimensions of the tube willbe matched to the cross-section of the resilient element.

The guide for the resilient element can comprise a tube, an arc-shapedstrip or both. However, the guide can also be formed in another way, forexample by means of a rail. The tube or the guide strip can be made of awear-resistant plastic, such as teflon.

According to the invention, it is also advantageous if the springconstant (C) of the resilient element decreases on further extension ofthe resilient element, preferably from a certain spring force (F) orextension (L). The equation F=C×L gives the relationship between thespring force, the spring constant and the extension here. If theresilient element has to become a little longer when the patient moveshis or her head, a resilient element of this type ensures that theadditional forces exerted on the teeth remain relatively small. This isparticularly advantageous when the head is not only turned to the leftor right but is also tilted a little up or down, such as, for example,when nodding to indicate yes. In this context, it is particularlyadvantageous, according to the invention, if, above a certain springforce or extension of the resilient element, the spring force of saidelement remains essentially constant on further extension thereof.Resilient elements of this type are known per se and can be produced,for example, from wires made of a so-called Nitinol alloy. Nitinol is aso-called super-elastic nickel-titanium alloy. Nitinol wires of thistype are known in orthodontics because of their particular elasticproperties and are fitted in the mouth as aids when correcting theposition of incisors or molars. Nitinol wires are described in, forexample, U.S. Pat. No. 4,037,324 and are described more extensively inthe literature references cited in column 7 of U.S. Pat. No. 4,037,324,which literature references are incorporated in the present Applicationby way of reference.

Nitinol wires are marketed by, amongst other companies, GACInternational Inc., New York, USA and TP Orthodontics Inc., La Porte,Ind., USA, under the trade names Sentalloy® and Reflex® respectively.

The invention also relates to an orthodontic aid provided with atensioning system according to the invention.

The invention will be explained in more detail below with the aid of anillustrative embodiment shown in the drawing. In the drawing:

FIG. 1 shows a perspective view of a tensioning system according to theinvention with an outer brace;

FIG. 2 shows a detail of a tensioning system according to the invention;

FIG. 3 shows a perspective view of a tensioning system according to theinvention with an outer brace: in this figure the enclosing tube for thetensioning system has been omitted;

FIG. 4 shows a detail of a tensioning system according to the invention,showing the fixing of the tensioning system to a neck band;

FIG. 5 shows a detail, partially in cross-section, of the fixing of thecoupling means to the outer brace; and

FIG. 6 shows, in diagrammatic longitudinal section, a detail of aparticularly advantageous embodiment of the tensioning system accordingto the invention; and

FIGS. 7a and 7b show graphs of spring characteristics of a helicalspring according to the invention made from Nitinol wire.

FIG. 1 shows a known outer brace, indicated by 21, with outer arms 1, towhich the inner arms 2 are fixed at 17. Bends are formed in the innerarms 2 at 3, the free ends of said bends being used for fixing the innerarms to the teeth. The outer brace 21 is provided, at its ends which arelocated outside the patient's mouth during use, with U-shaped hooks 7.

The tensioning system according to the invention comprises a helicalspring 5, which at its opposing ends is fixed to one end of cord 11,and, respectively, to one end of cord 12. The other ends of cords 11 and12, respectively, are clamped by means of a screw 10 in a shoe 9. Theshoes 9 are provided with safety hooks 8, which can be coupled to thehooks 7 of the outer brace 21.

FIG. 5 shows, in detail, the fixing of a safety hook 8 to a U-shapedhook 7, which is shown partially, in cross-section. The legs of thesafety hook are able to spread apart resiliently, so that the linkage isbroken if a certain tensile force, for example of 1000 gram, is exerted.

The safety hooks 8 are thus uncoupled if a pull of a certain force isexerted on the outer brace 1.

The spring 5 is fitted in a flexible tube 4, a detailed view of which isshown in FIG. 2.

FIG. 3 shows the tensioning system according to the invention togetherwith an orthodontic brace, the tube 4 being omitted in this figure. FIG.3 clearly shows the arc-shaped guide 13 in the form of a metal stripwhich has an essentially smooth sliding plane. Stops 19, in the form ofsmall tubes through which the wires 11 and 12 pass, are fixed at theends of the guide path. Said stops 19 ensure that the spring can not runoff the guide path. The guide path 13 having a smooth sliding planematched to the shape of the spring makes it possible for the spring 5 tobe able to be moved smoothly along the guide path without jolts orvibrations. FIG. 3 also shows bush-shaped plugs 18, which can be fittedin the ends of the tube 4. Said plugs are provided with longitudinalholes for the wires 10 and 11.

FIG. 2 shows a detailed view of a tube 4 according to the invention.Said tube 4 is formed from a helical spring 15 which is covered on theoutside by a covering 14, for example a so-called shrink sleeve. Saidcovering 14 ensures that none of the patient's hair can get caughtbetween the windings of the helical spring 15. The whole easily adjuststo the shape of the patient's neck.

The tensioning system according to the invention fitted in the tube 4can, as can be seen from FIG. 4, be fixed by means of, for example, aVelcro fastening to a neck band 6, which is known per se. Said neck band6 to some extent distributes the pressure over the neck and can easilybe replaced when it has become soiled.

The way in which the tensioning system according to the invention whichis shown in the drawing is fitted on the patient largely corresponds tothe fitting method as described in U.S. Pat. No. 3,526,035. The innerarms are placed in the mouth, the tensioning system is placed on theneck and the hooks 8 and 7 are engaged on either side of the patient'shead. The orthodontist can adjust the pre-tensioning of the spring 5 bymeans of the screws 10 which clamp the cords 11 and 12 in the shoes 9.

When fitting the tensioning system and the brace, the tube 4 adapts,because of its flexibility, to the shape of the patient's neck.Providing it is sufficiently flexible, the tube also continues to adjustduring use to the shapes of the neck, which, for example, is verypleasant when the patient is sleeping.

In FIG. 1, arrows show, diagrammatically, the functioning of thetensioning system according to the invention. In this figure, thepatient is imagined to have turned his or her head to the right inaccordance with arrow R. As a result of this movement, the centre of thespring 5 has moved in accordance with arrow r. The distance r over whichthe centre of the spring 5 has moved is equal to the difference betweenthe distances b and a, which show the respective distances between theshoes 9 and stops 18. The entire unit comprising the outer arms 1 of theouter brace 21, cord 11, spring 5 and cord 12, thus forms, as it were, a"ring" which partially passes through the tube 4, which "ring" turns asthe patient's head is turned, whilst the tube 4, which is fixedrelatively firmly to the patient's neck, remains in place unchanged. Itwill be clear that with this arrangement the tensioning force of thespring can remain constant because the spring 5 slides along the guide13 through the tube 4. If the head is turned in a purely rotarymovement, R is equal to r.

The friction between, on the one hand, the arc-shaped guide 13 and theresilient element 5 and, on the other hand, between the arc-shaped guideand the coupling element 11, such as a plastic cord, is very small withthis arrangement, so that said friction has hardly any influence on theforces exerted on the teeth. This is highly advantageous from thestandpoint of orthodontic considerations and for comfort in wear.

FIG. 6 shows, diagrammatically, an embodiment of the tensioning systemin which a metal spring 5 is able to move with low friction, withoutshocks and virtually soundlessly, i.e. barely audibly, over a metalguide path 13. To this end, plastic sleeves 30 are fitted around thespring 5 and sleeves 31, which are closed at one end and to which thecords 11 and 12 are also fixed, are fixed to the ends of the spring. Thesleeves 30, 31 prevent direct metal-on-metal contact and make itpossible for the spring to move smoothly backwards and forwards alongthe guide path. Smoothing down the sleeves 30, and preferably also thesleeves 31, somewhat on the side where said sleeves make contact withthe guide path 13 prevents a screw movement of the spring with respectto the sleeves 30 from being able to take place, so that said sleeves 30do not start to move along the spring.

FIGS. 7a and 7b show spring characteristics of two different helicalsprings made of Nitinol wire. In these graphs the abscissa shows theelongation Δl in mm, whilst the ordinate shows the spring force F in N.Both springs had an internal diameter of 2.0 mm and an external diameterof 2.8 mm. The spring used for FIG. 7a had an unloaded length of 15 mmand the spring used for FIG. 7b had an unloaded length of 20 mm. Thearrows shown on the curves in FIGS. 7a and 7b indicate the direction ofmovement of the spring. In these graphs an arrow pointing to the leftindicates the spring characteristic during extension, whilst an arrowpointing to the right indicates the spring characteristic when the loadon the spring is relaxed.

It can be seen that, for the spring in FIG. 7a above a spring force ofabout 3 N (about 300 g) and from an extension of about 30 mm, the springconstant decreases on further extension, which signifies that thegradient of the plot of the spring force against the extension becomessmaller. From an extension of 30 mm, which corresponds to a spring forceof 3 N, this spring can be subjected to substantial additionalextensions, for example up to 80 mm, whilst the spring force increasesby only about 1 N.

Something similar applies in the case of the spring constant for thespring from FIG. 7b. However, the boundary in this case is at a springforce of about 3.8 N and an extension of about 70 mm.

Springs having this type of spring characteristic, as described above,can be used highly advantageously in a tensioning system according tothe invention. After all, a specific spring force, which can be utilisedfor correction of the teeth, has to be achieved with a relatively smallextension of the spring, relatively little of the freedom of movementover the guide path being lost. If, when certain movements are made,such as a nodding movement to indicate yes, the circumference of the, asit were, "closed ring", has to increase, springs of this type can besubjected to a relatively large extension with a relatively smallincrease in the spring force, as a result of which the additional forcesexerted on the teeth remain limited. The patient's freedom of movementand the comfort in wear can be appreciably improved by this means.

When the patient's head makes more complex movements and noddingmovements to indicate yes, the spring 5 will frequently also have toextend in order to make these movements possible. The forces exerted onthe teeth will change as a result, which is adverse for the treatmentand reduces comfort in wear. This can be counteracted or prevented byusing, as the resilient element, an element for which the spring forceincreases only slightly or does not increase at all on further extensionof the spring.

It is pointed out that the tensile forces which are to be exerted on theteeth and are customary and desired in orthodontics are generally lessthan 500 gram. These tensile forces are frequently 100-150 gram andsometimes 300-400 gram. It will be clear that with such relatively smallforces the effect of friction can easily become apparent. Friction musttherefore be very low.

If the element used as the resilient element is an element for which thespring force remains essentially constant after a specific spring forceis reached, or for which the spring constant decreases on furtherextension after a specific spring force or extension is reached, it willbe possible to take the abovementioned tensile forces customary inorthodontics as the so-called specific spring force. Said specificspring force will then in general be less than 500 gram. Depending onthe correction of the teeth to be carried out and on the circumstances,this spring force will frequently be between 100 and 400 to 500 gram. Inpractice, tensile forces of 200 or 400 gram are found to be highlyadvantageous.

As can be seen from FIG. 1, the spring 5 can move freely to the right orto the left in the longitudinal direction of the tube 4. Because thecoupling elements 11 and 12 extend continuously from the ends of thespring 5 to the shoes 9 fitted to the hooks 7 and 8 of the outer brace,a maximum freedom of movement for the patient's head is obtained. Afterall, the spring can, in principle, be moved unhindered along the entireguide path. The patient can, therefore, turn his or her head freely andunhindered in a movement of shaking the head to indicate no. With thisarrangement, the tensile forces to be exerted on the teeth will remainessentially constant, until the spring 5 or a shoe 9 comes to restagainst a stop 19 or, respectively, a stop 18. When, as will preferablybe the case, the resilient element has a spring constant which decreasesafter a specific extension or spring force is reached, the increase inthe tensile force which occurs on one side of the teeth after thiscontact is made will be restricted or relatively small. A spring of thistype for which the spring constant decreases after a specific extensionor tensile force is reached also increases the comfort in wear and thefreedom of movement with regard to nodding movements of the head toindicate yes, during which such a spring will necessarily be extended.

It will be clear that many variants of the tensioning system accordingto the invention are conceivable, such as, for example:

the resilient element can be, for example, an elastic band, etc.;

the guide path can be formed by an arc-shaped strip, a tube, a rail,etc., the important factor here being mainly that the resilient elementcan be moved along without jerks;

cords, cables, strips, etc. can be used as the coupling means;

the flexibility of the tube 4 can also be achieved in some other waythan by means of a spiral-shaped winding 15;

the spiral-shaped winding 15 can be made of metal, plastic and the like.The lighter the weight, the better;

the guide path 13 can also be made of a plastic, which is preferablywear-resistant and causes minimal friction;

a resilient element having the resilient special characteristics asexplained in this Application, such as a helical spring made of Nitinolwire, can also be used as an immovable tensioning means. Some of theadvantages of the invention will then be lost.

I claim:
 1. A tensioning system for an extra-oral orthodontic outerbrace comprising an extra-oral tensioning element for exerting a tensileforce, wherein said element has a spring constant that decreases whensaid element is extended beyond a predetermined length and a neck bracein which said element is carried for being worn on a neck of a wearer.2. An orthodontic device comprising an extra-oral outer brace and atensioning system for exerting a tensile force on said outer brace, saidtensioning system comprising a coil spring of super-elasticnickel-titanium alloy wire connected to said outer brace.
 3. The deviceof claim 2, wherein said tensioning system further comprises a neckbrace in which said coil spring is carried.